Incorporation of aqueous reaction kinetics and biodegradation into TOUGHREACT: Application of a multi-region model to hydrobiogeoChemical transport of denitrification and sulfate reduction

The need to consider aqueous and sorption kinetics and microbiological processes arises in many subsurface problems. A general-rate expression has been implemented into the TOUGHREACT simulator, which considers multiple mechanisms (pathways) and includes multiple product, Monod, and inhibition terms. This paper presents a formulation for incorporating kinetic rates among primary species into mass-balance equations. The space discretization used is based on a flexible integral finite difference approach that uses irregular gridding to model bio-geologic structures. A general multi-region model for hydrological transport interacted with microbiological and geochemical processes is proposed. A 1-D reactive transport problem with kinetic biodegradation and sorption was used to test the enhanced simulator, which involves the processes that occur when a pulse of water containing NTA (nitrylotriacetate) and cobalt is injected into a column. The current simulation results agree very well with those obtained with other simulators. The applicability of this general multi-region model was validated by results from a published column experiment of denitrification and sulfate reduction. The matches with measured nitrate and sulfate concentrations were adjusted with the interficial area between mobile hydrological and immobile biological regions. Results suggest that TOUGHREACT can not only be a useful interpretative tool for biogeochemical experiments, but also can produce insight into processes and parameters of microscopic diffusion and their interplay with biogeochemical reactions. The geometric- and process-based multi-region model may provide a framework for understanding field-scale hydrobiogeochemical heterogeneities and upscaling parameters.